Square root calculation circuits and various uses of same are well documented. One such use is in telephone signalling receivers wherein tone detection is achieved by cross-correlation. One example of such a receiver is that described by Theo A. C. M. Cloosen and J. B. H. Peek in a publication of IEEE Transactions on Communication, Vol. Com-24, No. 12, December 1976 and entitled "A Digital Receiver for Tone Detection Application". Traditionally in telephone systems the telephone digit receiver is connected to service a calling telephone set during a primary signalling dialling portion of a telephone call progress. Once digits of a directory number have been received the digit receiver has traditionally been released so that it is free to be utilized during other call progresses as these occur. Recently expanded telephone services, for example voice messaging, have been introduced in some telecommunications systems. Various features of the voice messaging service are activated subsequent to the primary signalling or dialling portions of a call progress by yet further pushbutton generated dialling. This subsequent dialling is usually referred to as secondary signalling. Therefore in a telephone exchange having one or more such features, it is essential that each telephone call be monitored throughout its entirety, firstly for a short time for primary signalling and subsequently for the remainder of the call for secondary signalling. Consequently the cost of providing sufficient receivers in switching exchanges which are intended to accept secondary signalling has multiplied in comparison to those switching exchanges which accept only primary signalling.
Dramatic cost reductions in receivers have been achieved by speeding up the digit receiver function so that a digit receiver may be time shared among a plurality of active telephone calls. One example of a fast digit receiver is that disclosed by D. G. Agnew et al in U.S. Pat. No. 4,363,100 issued Dec. 7, 1982 and in Canadian Pat. No. 1,145,848 issued May 3, 1983. These patents are each entitled "Detection of Tones in Sampled Signals". The disclosed receiver produces a signal at its output which is porportional to a square root of a sum of two signals from respective branches of a time shared fast digital correlator circuit. The output signal is thus in a preferred reduced form for processing by a microcomputer suitably programmed for the signalling receiver function.